U.S. patent number 5,333,216 [Application Number 08/120,234] was granted by the patent office on 1994-07-26 for optical device used wavelength selective photocoupler.
This patent grant is currently assigned to Canon Kabushiki Kaisha. Invention is credited to Hitoshi Oda, Hajime Sakata.
United States Patent |
5,333,216 |
Sakata , et al. |
July 26, 1994 |
**Please see images for:
( Certificate of Correction ) ** |
Optical device used wavelength selective photocoupler
Abstract
An optical device comprises a substrate, a first guiding layer
having a guided mode provided on the substrate, a second guiding
layer laminated with the first guiding layer on the substrate, the
second guiding layer having a guided mode different from that of
the first guiding layer, and a diffraction grating provided on an
area where the first and second guiding layers have their guided
modes overlapping, the diffraction grating serving to couple the
light on the specific range of wavelengths that propagate through
the first guiding layer, with the second layer, wherein the
diffraction grating comprises a high refractive index region and a
low refractive index region disposed periodically in the
propagation direction of light, the ratio of the high refractive
index region to the low refractive index region occupied in one
period changing gradually in the propagation direction of
light.
Inventors: |
Sakata; Hajime (Hiratsuka,
JP), Oda; Hitoshi (Sagamihara, JP) |
Assignee: |
Canon Kabushiki Kaisha (Tokyo,
JP)
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Family
ID: |
26385042 |
Appl.
No.: |
08/120,234 |
Filed: |
September 14, 1993 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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621990 |
Dec 4, 1990 |
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Foreign Application Priority Data
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Dec 4, 1989 [JP] |
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1-314841 |
Feb 26, 1990 [JP] |
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2-45084 |
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Current U.S.
Class: |
385/28;
385/131 |
Current CPC
Class: |
G02B
6/12007 (20130101); G02B 6/124 (20130101); H01S
5/1032 (20130101); H01S 5/5045 (20130101); G02B
2006/12107 (20130101); G02B 2006/12109 (20130101); G02B
2006/12147 (20130101); H01S 5/0264 (20130101); H01S
5/1035 (20130101); H01S 5/1225 (20130101); H01S
5/3428 (20130101) |
Current International
Class: |
G02B
6/12 (20060101); G02B 6/124 (20060101); H01S
5/50 (20060101); H01S 5/10 (20060101); H01S
5/00 (20060101); H01S 5/34 (20060101); H01S
5/026 (20060101); H01S 5/12 (20060101); G02B
006/00 (); G02B 006/36 () |
Field of
Search: |
;385/27,28,37,129,130,131 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0187979 |
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Jul 1986 |
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EP |
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61-025607 |
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Feb 1986 |
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JP |
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1-107214 |
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Apr 1989 |
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JP |
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Other References
"Grating Assisted InGaAsp/InP Vertical Co-Directional Coupler
Filter" by Alferness et al, Technical Digest Series vol. 4, pp.
215-218 (1989). .
"Tunable Optical Waveguide Directional Coupler Filter," by R. C.
Alferness et al., Applied Physics Letters, vol. 33, No. 2, pp.
161-163 (Jul. 15, 1978). .
"Filter Characteristics of Codirectionally Coupled Waveguides with
Weighted Coupling," by R. C. Alferness et al., IEEE Journal of
Quantum Electronics, vol. QE-14, No. 11, pp. 843-847 (Nov. 1978).
.
Journal of Lightwave Technology, vol. LT-5, No. 2, Feb., 1987, New
York, N.Y., USA, pp. 268-273. .
Optical Engineering, vol. 19, No. 4, Aug., 1980, Bellingham, U.S.,
pp. 581-586..
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Primary Examiner: Ullah; Akm E.
Attorney, Agent or Firm: Fitzpatrick, Cella, Harper &
Scinto
Parent Case Text
This application is a continuation of application Ser. No.
07/621,990 filed Dec. 4, 1990, now abandoned.
Claims
We claim:
1. An optical device comprising:
a substrate;
a first guiding layer provided on said substrate;
a second guiding layer laminated with said first guiding layer on
said substrate, said second guiding layer having a guided mode
different from that of said first guiding layer; and
a diffraction grating provided on an area in which said first and
second guiding layers have their guided modes overlapping, said
diffraction grating serving to couple the light on the specific
range of wavelengths that propagates through said first guiding
layer, with said second guiding layer,
wherein said diffraction grating comprises a high refractive index
region and a low refractive index region disposed periodically in
the propagation direction of light, the ratio of the high
refractive index region to the low refractive index region occupied
in one period changing gradually in the propagation direction of
light.
2. An optical device according to claim 1, wherein said diffraction
grating is formed such that the coupling coefficient between the
first guiding layer and the second guiding layer is high in the
central portion of the diffraction grating in the propagation
direction of light, and is low on both end portions of the
diffraction grating.
3. An optical device according to claim 2, wherein said diffraction
grating is formed such that the coupling coefficient between the
first guiding layer and the second guiding layer is symmetrically
distributed with respect to the central portion of the diffraction
grating in the propagation direction of light.
4. An optical device according to claim 2, wherein said diffraction
grating is formed such that the ratio of the high refractive index
region to the low refractive index region occupied one period is
high in the central portion of the diffraction grating in the
propagation direction of light, and is low on both end portions of
the diffraction grating.
5. An optical device according to claim 4, wherein said diffraction
grating consisting of a corrugation formed on the second guiding
layer, and is formed such that the ratio of a ridge portion of the
corrugation to a groove portion of the corrugation occupied in one
period is high in the central portion of the diffraction grating
int eh propagation direction of light, and is low on both end
portions of the diffraction grating.
6. An optical device according to claim 2, wherein when the
complete coupling length between the first guiding layer and the
second guiding layer is L, the progressive direction of light to be
the z direction, the coupling coefficient which changes in the z
direction to be G(z), a taper function for the coupling coefficient
distribution G(z) to be F(z), and G.sub.0 to be a constant, the
following relation is satisfied; ##EQU5##
7. An optical device according to claim 6, wherein said taper
function F(z) is a Hamming function as expressed in the
following:
8. An optical device according to claim 6, wherein said taper
function is a Blackman function as expressed in the following:
9. An optical device according to claim 6, wherein said taper
function is a raised cosine function as expressed in the
following:
10. An optical device according to claim 6, wherein said taper
function is a Kaiser function as expressed in the following:
where .gamma. is an arbitrary number, and I.sub.0 is the zero order
value of the Bessel function of the first kind.
11. An optical device according to claim 1, wherein, when the
wavelength of said propagating light is .lambda., the propagation
constant n the guided mode for the first guiding layer is
.beta..sub.0 (.lambda.), the propagation constant in the guided
mode for the second guiding layer is .beta..sub.1 (.lambda.), and
the pitch of the diffraction grating is .LAMBDA., the following
expression is satisfied,
12. An optical device according to claim 1, further comprising
first, second, and third cladding layers, wherein said first
cladding layer, said first guiding layer, said second cladding
layer, said second guiding layer, and said third cladding layer are
sequentially laminated on said substrate.
13. An optical device according to claim 7, wherein said substrate
and each of said layers consist of GaAs or AlGaAs.
14. An optical device according to claim 13, wherein said first and
second guiding layers have multiple quantum wells.
15. An optical device according to claim 1, further comprising a
light absorption layer for absorbing at least a part of the light
coupled with the second guiding layer and an electrode for
converting the light absorbed in said light absorption layer into
an electrical signal to be outputted therefrom.
16. An optical device according to claim 15, wherein said light
absorption layer is arranged in series with the second guiding
layer with respect to the light propagation direction.
17. An optical device according to claim 15, further comprising an
n-type semiconductor layer and a p-type semiconductor layer which
sandwich the light absorption layer therebetween, and wherein said
light absorption layer comprises an i-type semiconductor layer.
18. An optical device according to claim 1, wherein said second
guiding layer absorbs the light coupled with said second guiding
layer by the diffraction grating, and said device further comprises
an electrode for converting the light absorbed in the second
guiding layer into an electrical signal to be outputted
therefrom.
19. An optical device according to claim 18, further comprising
first, second and third cladding layers, and wherein the first
cladding layer, first guiding layer, second cladding layer, second
guiding layer and third cladding layer are sequentially laminated
on said substrate.
20. An optical device according to claim 19, wherein each of said
first guiding layer, first and second cladding layers comprise a
semiconductor having a first conductive type, said third cladding
layer comprises a semiconductor having a second conductive type,
and said second guiding layer comprises an i-type
semiconductor.
21. An optical device according to claim 20, wherein said first
conductive type is an n-type and said second conductive type is a
p-type.
22. An optical device according to claim 19, wherein said substrate
comprises a semi-insulating semiconductor, said first and second
guiding layers, said first, second and third cladding layers
respectively comprise i-type semiconductors, and wherein a
stripe-like region extending in the light propagation direction is
formed in a part of the lamination of the substrate, first and
second guiding layers, the first, second and third cladding layers,
and a p-type region and an n-type region are provided on both sides
of said stripe-like region.
23. An optical device according to claim 19, further comprising a
source electrode and drain electrode which contact with said p-type
region and n-type region, respectively, an insulating layer formed
on said third cladding layer of the stripe-like region, and a gate
electrode provided on said insulating layer.
24. An optical device according to claim 1, further comprising a
laser active region provided on at least a part of said second
guiding layer and an electrode for supplying a current to said
laser active region, and wherein said laser active region amplifies
the light propagating through said second guiding layer by
supplying the current thereinto.
25. An optical device according to claim 24, wherein said
diffraction grating comprises a first diffraction grating and a
second diffraction grating which are separately provided at a
predetermined interval in the light propagation direction, and
wherein the light propagating through said first guiding layer is
coupled with said second guiding layer by said first diffraction
grating and the light coupled and propagated in said second guiding
layer is coupled with said first guiding layer again by said second
diffraction grating.
26. An optical device according to claim 25, wherein an impurity is
diffused in a portion of said second guiding layer other than a
region between said first and second diffraction gratings.
27. An optical device according to claim 24, further comprising a
first, second and third cladding layers, and wherein said first
cladding layer, first guiding layer, second cladding layer, second
guiding layer and third cladding layer are sequentially laminated
on said substrate.
28. An optical device according to claim 27, wherein said first
cladding layer, first guiding layer, second cladding layer, second
guiding layer, and third cladding layer are mesa-etched except for
said stripe-like region extending in the light propagation
direction, and an embedded layer is formed on both sides of the
mesa region.
29. An optical device according to claim 27, wherein a stripe-like
ridge extending in the light propagation direction is formed on
said second cladding layer, second guiding layer and third cladding
layer by etching.
30. An optical device comprising:
a substrate;
a first guiding layer provided on said substrate and having a
guided mode;
a second guiding layer laminated with said first guiding layer on
said substrate, said second guiding layer having a guided mode
different from the guided mode of said first guiding layer; and
a diffraction grating provided on an area in which said first and
second guiding layers have their guided modes overlapping, said
diffraction grating serving to couple the light in the specific
range of wavelengths that propagate through said first guiding
layer, with said second guiding layer,
wherein said diffraction grating comprises a high refractive index
region and a low refractive index region disposed periodically in
the propagation direction of light, the ratio of the high
refractive index region to the low refractive index region changing
gradually in the propagation direction of light, and the pitch of
said diffraction grating gradually changing in the propagation
direction of light.
31. An optical device according to claim 30, wherein said
diffraction grating is formed such that the coupling coefficient
between the first guiding layer and the second guiding layer is
high in the central portion of the diffraction grating in the
propagation direction of light, and is low on both end portions of
the diffraction grating.
32. An optical device according to claim 31, wherein said
diffraction grating is formed such that the coupling coefficient
between the first guiding layer and the second guiding layer is
symmetrically distributed with respect to the central portion of
the diffraction grating in the propagation direction of light.
33. An optical device according to claim 31, wherein said
diffraction grating is formed such that the ratio of the high
refractive index region to the low refractive index region occupied
in one period is high in the central portion of the diffraction
grating in the propagation direction of light, and is low on both
end portions of the diffraction grating.
34. An optical device according to claim 33, wherein said
diffraction grating consists of a corrugation formed on the second
guiding layer, and is formed such that the ratio of the ridge
portion of the corrugation to a groove portion of the corrugation
occupied in one period is high in the central portion of the
diffraction grating in the propagation direct on of light, and low
on both end portions of the diffraction grating.
35. An optical device according to claim 30, wherein when the
wavelength of said propagating light is .lambda., the propagation
constant in the guided mode for the first guiding layer is
.beta..sub.0 (.lambda.), the propagation constant in the guided
mode for the second guiding layer is .beta..sub.1 (.lambda.), and
the pitch of the diffraction grating is .LAMBDA., the following
expression is satisfied,
36. An optical device according to claim 30, further comprising
first, second, and third cladding layers, wherein said first
cladding layer, said first guiding layer, said second cladding
layer, said second guiding layer, and said third cladding layer are
sequentially laminated on said substrate.
37. An optical device according to claim 30, wherein said substrate
and each of said layers consist of GaAs for AlGaAs.
38. An optical device according to claim 37, wherein said first and
second guiding layers have multiple quantum wells.
39. An optical device according to claim 31, wherein the following
relations are satisfied: ##EQU6## where L is the complete coupling
length between the first guiding layer and the second guiding
layer, G(z) is the coupling coefficient of light in the z
direction, F(z) is the taper function for distributing the coupling
coefficient G(z), and G.sub.0 is a constant.
40. An optical device according to claim 39, wherein said coupling
coefficient has a distribution approximating a Hamming function
F(z) as expressed in the following, in the propagation direction of
light, i.e., the z direction,
where L is the complete coupling length between the first guiding
layer and the second guiding layer.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical device such as an
optical filter, photodetector, semiconductor laser, or optic
amplifier, using a wavelength selective photocoupler comprising two
waveguides coupled by a diffraction grating.
2. Related Background Art
A conventional wavelength selective photocoupler consisted of two
optical waveguides formed on the same substrate, as described in R.
C. Alferness et al.: Applied Physics Letters, 33, P161 (1978),
Japanese Laid-Open Patent Application No. 61-250607, or Miki et
al., Electronic Communications Institute study report OQE81-129,
for example.
FIG. 1 shows the construction of such a conventional wavelength
selective photocoupler. In the same figure, two waveguides 191, 192
are formed with line widths or heights W.sub.1, W.sub.2, and
refractive indexes n.sub.1, n.sub.2, which are different from each
other, as shown. Thus, they have also different distribution
relations between the wavelength of guided light propagating
through the respective optical waveguides 191, 192 and the
propagation constant. The propagation constants of the two
waveguides coincide for guided light on a specific range of
wavelength, and the optical coupling takes place between the two
waveguides 191, 192. In other words, only the light within the
specific range of wavelength is selected, and the optical power can
be transferred between the waveguides 191, 192.
Such a photocoupler has been used for an optical filter which
performs the multiplexing or demultiplexing of waves between the
signal light and the light of specific wavelength by using the
transfer of this optical power.
However, the optical filter as above mentioned had the spectral
transmissivity characteristic containing the non-negligible side
lobe on both sides of the transmission band (main lobe) including a
central wavelength, as shown in FIG. 2. The existence of the side
lobe brought about the cross talk of optical signals when this
optical filter was used in the optical communication system using
the wavelength division multiplexing method. If there is provided a
sufficient separation of wavelengths to avoid the cross talk, the
number of communication channels is decreased. The side lobe of the
filtering characteristic on such a photocoupler was a factor in
preventing the enhancement of performance on an optical device
using this photocoupler.
A method for suppressing the side lobe as above mentioned, in which
an interval between waveguides 193, 194 constituting the
photocoupler is gradually changed as shown in FIG. 3, was proposed
in R. C. Alferness et al.: IEEE Journal of Quantum Electronics,
QE-14, No. 11, p. 843 (1978).
However, this method had a disadvantage that it was difficult to
form the waveguide 193 curvilinearly, and further difficult to
manufacture the photocoupler of a type of laminating waveguides in
the direction of thickness.
A photocoupler having a sharp wavelength selectively and little
loss of quantity of light was proposed in R. C. Alferness et al.,
"Grating Assisted in GaAsP/INP vertical co-directing coupler
filter" 1989 technical digest series vol. 4, pp 215-218. This
photocoupler had two guiding layers with different guided modes
from each other laminated on a substrate and coupled optically by a
diffraction grating. The present inventor has proposed an optical
device using this photocoupler in U.S. Ser. No. 491,203 filed on
Mar. 9, 1990.
SUMMARY OF THE INVENTION
An object of this invention is to provide an optical device which
operates more efficiently by further improving the above mentioned
wavelength selective photocoupler using a diffraction grating, and
suppressing the side lobe of the filtering characteristic.
The above object can be accomplished by an optical device
comprising:
a substrate;
a first guiding layer provided on said substrate;
a second guiding layer laminated with said first layer on said
substrate, said second guiding layer having a guided mode
difference from that of said first layer; and
a diffraction grating provided on a region where said first and
second guiding layers have their guided modes overlapping, said
diffraction grating serving to couple the light within the specific
range of wavelength that propagates through said first layer with
said second layer,
wherein said diffraction grating comprising high refractive index
regions and low refractive index regions disposed periodically in
the propagation direction of light, the ratio of the high
refractive index regions to the low refractive index regions
changing gradually in the propagation direction of light.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view showing the first example of a
conventional wavelength selective photocoupler,
FIG. 2 is a view showing the spectral transmissivity characteristic
of an optical filter using the example of FIG. 1,
FIG. 3 is a schematic view showing the second example of a
conventional wavelength selective photocoupler,
FIG. 4 is a schematic cross-sectional view showing view showing the
first example of an optical device according to this invention,
FIG. 5 is a view showing the optical electric field distribution
for each guided mode in the first example,
FIG. 6 is a view showing a relation between the groove
width/grating pitch and the coupling coefficient,
FIG. 7 is a view showing a spatial change of the coupling
coefficient for the grating in the first example,
FIG. 8 is a view showing the filter spectrum in the first
example,
FIG. 9 is a view showing the filter spectrum of a conventional
diffraction grating,
FIG. 10 is a schematic perspective view showing the second example
of an optical device according to this invention,
FIG. 11 is a view showing a spatial change of the coupling
coefficient for the grating in the second example,
FIG. 12 is a view showing the filter spectrum in the second
example.
FIG. 13 is a cross-sectional side view showing the third example of
an optical device according to this invention,
FIG. 14 is a view showing the wavelength characteristic of electric
signal detected in the third example,
FIG. 15 is a cross-sectional side view showing the fourth example
of an optical device according to this invention,
FIG. 16 is a view showing the filter spectrum in the fourth
example,
FIG. 17 is a schematic perspective view showing the fifth example
of an optical device according to this invention,
FIG. 18 is a schematic perspective view showing the sixth example
of an optical device according to this invention,
FIG. 19 is a cross-sectional side view showing the seventh example
of an optical device according to this invention,
FIG. 20 is a cross-sectional front view showing the seventh example
of an optical device according to this invention,
FIG. 21 is a cross-sectional front view showing the eighth example
of an optical device according to this invention,
FIG. 22 is a cross-sectional side view showing the eighth example
of an optical device according to this invention,
FIG. 23 is a cross-sectional side view showing the ninth example of
an optical device according to this invention,
FIG. 24 is a partial cross-sectional perspective view showing the
tenth example of an optical device according to this invention,
FIG. 25 is a view showing the distribution of coupling coefficient
in the propagation direction of light in the tenth example,
FIG. 26 is a view showing the relations between the duty ratio the
pitch and the coupling coefficient in the tenth example,
FIG. 27 is a view showing the change of the duty ratio in the
propagation direction of light in the tenth example,
FIG. 28 is a view showing the change of the grating pitch in the
propagation direction of light in the tenth example,
FIG. 29 is a view showing the characteristic of the band-pass
filter in the tenth example,
FIG. 30 is a view showing the characteristic of the notch filter in
the tenth example,
FIGS. 31 and 32 are views showing other examples of the relations
between the duty ratio of grating, the pitch and the coupling
coefficient,
FIG. 33 is a view showing the relation between the grating depth
and the coupling length,
FIG. 34 is a cross-sectional side view showing the eleventh example
of an optical device according to this invention, and a view
showing the change of the coupling coefficient, and
FIG. 35 is a view showing the characteristic of the band-pass
filter in the eleventh example.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to FIG. 4, the construction of the first example
according to this invention is depicted. The present example has
grown in sequence, with the molecular beam epitaxy (MBE), on a
substrate 1 of GaAs, a buffer layer 2 of GaAs, 0.5 .mu.m thick, a
cladding layer 3 of Al.sub.0.5 Ga.sub.0.5 As, 1.5 .mu.m thick, a
first waveguide 4, 0.1 .mu.m thick, which is made a multiple
quantum well (MQW) by laminating GaAS and Al.sub.0.4 Ga.sub.0.6 As,
alternately, a cladding layer 5 of Al.sub.0.5 Ga.sub.0.5 As, 0.8
.mu.m thick, and a second layer 6, 0.4 .mu.m thick, which is made a
MQW by laminating GaAs and Al.sub.0.2 Ga.sub.0.8 As, alternately.
Then, after making a resist mask with the photolithography method
using a photoresist, a grating 7 consisting of corrugation 0.07
.mu.m in depth is formed in portions on an upper surface of the
second waveguide 6 with reactive ion beam etching (RIBE).
In this example, in order to suppress the side lobe of the filter
spectrum, the coupling coefficient of the grating 7 is changed
almost symmetrically along the traveling direction of light from
its central portion toward the incident and propagation sides of
the light. In other words, the pattern of the grating 7 is changed,
as shown in FIG. 4, by making the pitch .LAMBDA. constant and
changing the proportion of ridge to groove (or line to space)
constituting the grating 7 along the progressive direction of
light. The ridge is a portion of high refractive index n.sub.2,
while the groove is a portion of low refractive index n.sub.1.
After forming the grating 7, the SiO.sub.2 layer 8 is coated on the
grating 7 with the sputtering method. Then after applying a
photoresist again, and making a pattern of stripe extending in the
propagation direction of light for laterally confining the guided
light, a wafer is etched with the RIBE method, to a position where
the GaAs substrate 1 is exposed, in which a leading edge of stripe
is cut obliquely to the progressive direction of light to couple
the light to only the first waveguide 4 when the light enters.
Then Al.sub.0.5 Ga.sub.0.5 As is grown, with the liquid phase
epitaxy (LPE) method, to endbed the stripe, and SiO.sub.2 is coated
again over the entire surface. Consequently, the structure with two
waveguides 4, 6 laminated as shown in FIG. 4 can be obtained.
In this way, the optical wavelength filter of this example has two
layers of waveguides (first waveguide 4, second waveguide 6)
laminated in the layer direction, thereby constituting a
directional coupler. As each waveguide 4, 6 is formed to have a
different thickness or composition, it has a different propagation
constant for light propagating through it. The grating 7 formed on
the second waveguide 6 is used to select a wavelength to be coupled
optically. The wavelength can be selected by changing the pitch or
the ratio of ridge to groove.
FIG. 5 shows the optical electric field distribution in the guided
modes of this example. The longitudinal axis is an optical electric
field intensity distribution, while the transversal axis is a
distance in the lamination direction with reference to an upper
surface of the second waveguide 6. Thus, there are two guided modes
in the waveguides 4, 6 of this example, i.e., the odd mode 11 which
stands on the first waveguide 4 as the central portion, and the
even mode 12 which stands on the second waveguide 6 as the central
portion. The grating 7 is formed in an overlapping portion (left
portion of the second waveguide 6 in FIG. 5) of the odd mode 11 and
the even mode 12, as above described.
The operation of this example will be described.
The incident light 14 multiplexed in wavelengths ranging from 0.80
.mu.m to 0.86 .mu.m is coupled for input to the first waveguide 4,
as shown in FIG. 4. There are two guided modes which stand in two
waveguides 4, 6, i.e., the even mode 12 and the odd mode 11, as
previously described. The incident light 14 input to the first
waveguide 4 propagates in the odd mode 11 that stands on the first
waveguide 4 as the central portion. As the odd mode 11 and the even
mode 12 have different propagation constants, the light propagates
nearly independently almost without coupling at the area where the
grating 7 does not exist. However, at the area where the grating 7
exists, the transfer of optic power occurs if the following
relation exists between the propagation constant .beta..sub.odd for
the odd mode 11 and the propagation constant .beta..sub.even for
the even mode 12.
Where .lambda. is the wavelength of light, and .LAMBDA. is the
pitch of grating 7.
If the transfer of optic power occurs as described above, the
guided light in the odd mode 11 to which the incident light 14 is
coupled can be converted into the guided light in the even mode 12.
Accordingly, the incident light 14 ultimately becomes the optical
wave propagating through the second waveguide 6, and is output as
the selected output light 15. The light with other wavelengths is
output from the first waveguide 4 as the unselected output light
16.
Also, a coupling coefficient g is provided, which is a coefficient
expressing the coupling intensity of optical waves propagating the
two waveguides 4, 6. The coupling coefficient g is given by the
following expression.
Where .epsilon..sub.even, .epsilon..sub.odd is the optical electric
field distribution in the even or odd mode, respectively, and
A.sub.1 is a component of Fourier series corresponding to the
primary diffracted light for the grating 7. Assuming that the ridge
and groove of the grating 7 is rectangular, A.sub.1 (x) is given by
the following expression: ##EQU1## where q=1 as the coupling of
.beta..sub.even and .beta..sub.odd is performed by the primary
diffracted light, n.sub.1 and n.sub.2 are refractive indices of
materials constituting the ridge and groove of the grating 7,
respectively, .LAMBDA. is a pitch of the grating 7, and w is a
groove width. In other words, the coupling coefficient g depends on
the ratio of ridge to groove occupied in the pitch .LAMBDA. of the
grating 7. FIG. 6 is a graph representing the change of the
coupling coefficient g when the ratio of ridge to groove in the
grating 7 is changed in a directional coupler type optical filter
in accordance with the configuration of this example. In the
example as shown, the intensity of coupling is at maximum near the
position where the ratio of ridge to groove is 0.75:0.25, and
decreases below or beyond that position. However, the ratio of
ridge to groove at which the intensity of coupling is at maximum
depends on the configuration of waveguide.
Thus, in this example, in order to perform the wavelength filtering
with the light having a central wavelength of 0.83 .mu.m, the pitch
.LAMBDA. is set to be 7.8 .mu.m according to the expression (1),
and the ratio of ridge to groove constituting the grating 7 is
configured as shown in FIG. 4. In other words, the ratio of ridge
to groove is 0.7:0.3 in the center of the grating area with the
groove gradually increasing in proportion toward the periphery, and
the ratio is 0.1:0.9 at both ends. In this way, the coupling
coefficient or the intensity of coupling is changed in the same way
as in the conventional example as shown in FIG. 3, by gradually
changing the ratio of ridge to groove form the center of grating
toward the periphery.
Here, the length of coupling or the complete coupling length is
defined to exemplify the distribution for the intensity of
coupling. Assuming that the progressive direction of guided light
is the z direction, the complete coupling length is L, and the
coupling coefficient which changes in the z direction is G(z), the
following expression is satisfied: ##EQU2## where the function for
distribution G(z) is referred to as a Taper function, which is set
to be F(z), and the following expression is obtained:
(G.sub.0 is a constant)
Note that F(z) is normalized with L, i.e., the following expression
is satisfied: ##EQU3## From the expressions (4), (5) and (6), the
complete coupling length L can be obtained as in the following:
##EQU4## As G.sub.O is 21.5 cm.sup.-1 in this example, the coupling
length L is 730 (.mu.m). Thus it is assumed in this example that
the coupling length L is 730 .mu.m, and the Taper function F(z) for
representing the distribution of the intensity of coupling is a
Hamming function as shown in FIG. 7. This Hamming function is
expressed as in the following:
With the above configuration, the intensity (filter spectrum) of
selected output light 15 to be output from the second waveguide 6
among the light 14 incident upon the first waveguide 4 is as shown
in FIG. 8. For comparison, the spectrum of one example is shown in
FIG. 9, in which the ratio of ridge to groove of the grating is
constant, and the intensity of coupling is made constant over the
whole area where the grating is formed. With the comparison between
FIG. 8 and FIG. 9, it should be clear that the suppression effect
of side lobes can be sufficiently obtained with the configuration
of this example. From the filter spectrum as shown in FIG. 8, it
will be seen that the full width at half maximum is 55.ANG. and the
ratio of the transmitted light intensity on the central wavelength
of main lobe to that on the wavelength 100.ANG. off that central
wavelength is about 30 dB. In the configuration of FIG. 4, it
should be noted that an unreflective coating of ZrO.sub.2 is
applied on the input and output end faces to suppress the decrease
of efficiency and the occurrence of ripples accompanied by the
reflection upon the end faces.
FIG. 10 shows a second example. The second example, in the same way
as the first example, has grown in sequence with the molecular beam
epitaxy (MBE), on a GaAs substrate 21, a buffer layer 22 of GaAs,
0.5 .mu.m thick, a cladding layer 23 of Al.sub.0.5 Ga.sub.0.5 As,
1.5 .mu.m thick, a first guiding layer 24, 0.15 .mu.m thick, which
is made a multiple quantum well (MQW) by laminating GaAs and
Al.sub.0.4 Ga.sub.0.6 As, alternately, a cladding layer 25 of
Al.sub.0.5 Ga.sub.0.5 As, 0.9 .mu.m thick, and a guiding layer 26,
0.5 .mu.m thick, which is made a MQW by laminating GaAs and
Al.sub.0.2 Ga.sub.0.8 As, alternately. Then with the photoresist
and RIBE method, the grating (not shown) consisting of corrugation
0.3 .mu.m deep is formed on the second guiding layer 26, and then
with the LPE method, the layer of Al.sub.0.5 Ga.sub.0.5 As is
regrown to embed the groove of the grating. Subsequently, the GaAs
layer is grown as a cap layer 28, and after forming the stripe with
SiO.sub.2, impurities such as Zn (or Si) are placed on both sides
of the stripe, with thermal diffusion, to confine the light in the
transverse direction. Thereby the first guiding layer 24 and the
second guiding layer 26 have both of their sides disordered, where
a region 29 with a low refractive index is formed, so that the
optical wavelength filter as shown in FIG. 10 is made.
With the low refractive index region 29 formed on both sides of the
first guiding layer 24 and the second guiding layer 26, the guided
light is confined in the transverse direction to reduce the loss
due to diffraction spreading of the guided light, which results in
a highly efficient optical wavelength filter.
The confinement in the transverse direction can be accomplished in
various ways, such as a method of forming ridges or a loading
method, besides the method as described in the first and second
examples.
In the second example, the ratio of ridge (high refractive index
region) to groove (low refractive index region) constituting the
grating is changed from 0.6:0.4 to 1.0 from the center to the
periphery to increase the ratio of ridge to groove.
With this configuration, the change of the coupling coefficient on
the grating area follows the shape of a Blackman function, as shown
in FIG. 11. The Blackman function F(z) is expressed as follows:
From the filter spectrum of the second example as shown in FIG. 12,
it is seen how the side lobe is suppressed. The full width of half
maximum is 73.ANG., and the ratio of the transmittivity on the
central wavelength the main lobe to that on the wavelength 100.ANG.
off that central wavelength is from 30 dB to 35 dB or more.
Further, besides the Taper function for the coupling coefficient in
the first and second examples, the following distribution is
provided. That is the coupling coefficient can be given in the
propagation direction of light by the raised cosine function,
or the Kaiser function
where L is the complete coupling length, z is a position along the
waveguide in -L/2.ltoreq.z.ltoreq.L/2, .gamma. is an arbitrary
number, and I.sub.0 is the zero order value of the Bessel function
of the first kind. The side lobe can be suppressed if the grating
with a distribution in accordance with the above function is
formed.
In the above examples, the grating is formed on the second
waveguide 6, 26, but the grating may be made at any portions where
the optical electric field distributions (even mode 12, odd mode
11) of the guided light both exist. However, since the coupling
coefficient changes with the coupling length L, the coupling length
L must be correspondingly adjusted.
Each of the above examples was constructed from GaAs/AlGaAs
materials, but it is clear that they can be constructed from other
materials, such as semiconductor compounds of InGaAs/InGaP, glass
materials of SiO.sub.2 /TiO.sub.2, or optical crystals of
LiNbO.sub.3, LiTaO.sub.3, and BSO.
FIG. 13 shows the third example according to this invention. This
example was embodied by integrating photodetectors having
wavelength selectiveness, with the construction of an optical
wavelength filter as previously described.
The third example has grown in sequence with the molecular beam
epitaxy (MBE), on a substrate 31 of N.sup.+ -GaAs, a buffer layer
32 of n-GaAs, 0.5 .mu.m thick, a cladding layer 33 of n-Al.sub.0.5
Ga.sub.0.5 As, 1.5 .mu.m thick, a first guiding 34 of n-Al.sub.0.3
Ga.sub.0.7 As, 0.2 .mu.m thick, a cladding layer 35 of n-Al.sub.0.5
Ga.sub.0.5 As, 0.8 .mu.m thick, and a second layer 36, 0.4 .mu.m
thick, which is made a MQW by laminating i-GaAs and i-Al.sub.0.4
Ga.sub.0.6 As, alternately. Then with the photolithography method,
the grating of corrugation, 0.05 .mu.m in depth and 7.7 .mu.m in
pitch with the ratio of ridge to groove changing in the traveling
direction of light in accordance with the Kaiser function
(expression (8) as shown above), was formed on an upper surface of
the second guiding layer 36 over the length 1.277 mm.
Then with the liquid phase epitaxy (LPE) method, a cladding layer
38 of i-Al.sub.0.5 Ga.sub.0.5 As, and a cap layer 39 of i-GaAs, 0.5
.mu.m thick, are regrown on that surface. Thereafter, the cladding
layer 38 and the second layer 36 in the region adjacent to the
grating 37 are removed by etching. Further, with the LPE method, an
absorption layer of i-GaAs, 0.1 .mu.m thick, a cladding layer 41 of
p-Al.sub.0.5 Ga.sub.0.5 As, 1.2 .mu.m thick, and a cap layer of
p.sup.+ -GaAs, 0.5 .mu.m thick are regrown on that removed area,
and subsequently, an electrode 43 consisting of Cr/Au is formed on
the cap layer 42, and an electrode 44 consisting of AuGe/Au is
formed on the back surface of the substrate 31.
In this example, only the light having the wavelength selected by
the optical wavelength filter from the light 46 incident upon the
first guiding layer 34 is coupled to the second guiding layer 36,
and absorbed in the absorption layer 40 which is a photodetection
portion. The photodetection portion has a p-i-n structure, and a
reverse bias is applied between electrodes 43, 44. Therefore,
carriers caused by the absorption are detected as the electric
current signal.
FIG. 14 is a view showing the wavelength characteristic of signal
light to be taken out as electric signals in this example. The full
width of half maximum, i.e., the bandwidth of the -3 dB which is a
wavelength selective characteristic, is about 78.ANG.. The ratio of
output current at the central wavelength of the main lobe to that
of the wavelength 100.ANG. off that central wavelength (or cross
talk between wavelengths) is about 30 dB, which indicates that the
side lobe is sufficiently suppressed.
FIG. 15 is a view showing the construction of the fourth example
according to this invention. This fourth example has grown in
sequence on the substrate 51 of n.sup.+ -GaAs, with the molecular
beam epitaxy (MBE), a buffer layer 52 of n-GaAs, 0.5 .mu.m thick, a
cladding layer 53 of n-Al.sub.0.5 Ga.sub.0.5 As, 1.5 .mu.m thick, a
guiding layer 54, 0.2 .mu.m thick, which is made a multiple quantum
well (MQW) by laminating n-GaAs, 30.ANG. thick, and Al.sub.0.5
Ga.sub.0.5 As, 70.ANG. thick, alternately, a cladding layer 55 of
n-Al.sub.0.5 Ga.sub.0.5 As, 0.7 .mu.m thick, and a light absorption
layer 56 of i-GaAs, 0.4 .mu.m thick.
Then with the photolithography method using a photoresist, a resist
mask is made, an upper surface of the light absorption layer 56 is
etched by using ammonia and hydrogen peroxide, and then the grating
57 consisting of corrugation is formed over the length 100 .mu.m,
in which the grating has a depth 0.2 .mu.m, a pitch 5.5 .mu.m, and
the ratio of ridge to groove of the grating 57 changing in the
traveling direction of light in accordance with the raised cosine
function as shown above in the expression (7).
Then with the liquid phase epitaxy (LPE) method, a cladding layer
58 consisting of p-Al.sub.0.5 Ga.sub.0.5 As is regrown, and further
a cap layer 59 consisting of p.sup.+ -GaAs is regrown. Finally, a
contact layer (not shown) consisting of Au-Ge and an electrode 60
of Au are coated on the back surface of the substrate 51, and a
contact layer (not shown) and an electrode 61 of Au are coated on
the upper surface of the cap layer 59. In this way, a photodetector
which is a p-i-n type photodiode is made. The photodetector of this
example is constructed as shown above so that the guiding layer 54
and the light absorption layer 56 laminated in the layer direction
form a directional photocoupler. As the guiding layer 54 and the
light absorption layer 56 have different compositions and
thicknesses of layers, the propagation constants of light
propagating through each layer are different. The grating 57 formed
on the upper surface of the light absorption layer 56 is used to
select the light with which the directional coupling is performed
by changing the pitch of grating and the ratio of ridge to
groove.
The operation of this example is described as follows. A reverse
bias is applied between electrodes 60, 61 of this example, and the
signal light 64 which consists of light having wavelengths ranging
from 0.78 .mu.m to 0.88 .mu.m at intervals of 0.01 .mu.m enters the
guiding layer 54 by the end face coupling. The input signal light
64 propagates in the odd mode 11 having the central intensity on
the guiding layer 54, wherein either the even or odd mode 11, 12
stands within the photocoupler as shown in FIG. 5. As the optical
electric intensity distribution of this odd mode 11 fails to reach
the light absorption layer 56 almost entirely, as shown in FIG. 5,
the propagation loss due to the absorption in the absorption layer
56 is quite small.
If the relation as expressed in (1) is satisfied for the specific
wavelength as above described, the light of the odd mode 11 is
converted into that of the even mode 12, and the central intensity
transfers to the light absorption layer 56 In this example the
grating pitch .LAMBDA. is 5.5 .mu.m and a wavelength of 0.83 .mu.m
is detected. Thus the guided light transferred into the light
absorption layer 56 is absorbed, creating electrons and holes,
which are detected externally as the photoelectric current. FIG. 16
is a view showing the wavelength distribution of light detected in
this example. It will be seen how the sharp selection is performed
with the side lobe sufficiently suppressed at the full width at
half maximum 64.ANG..
In this example, the grating 57 is formed over the length 100
.mu.m, which is a length not reaching to the complete coupling
length of 262 .mu.m (the length of the coupling area where the
coupling efficiency is at maximum) of a directional coupler. This
is because the responsibility of the photodetector has been taken
into consideration. Therefore, if the responsibility is degraded
due to the increased reception area of light, the absorption
efficiency of light can be further increased by making the length
of the grating area closer to the complete coupling length.
If a plurality of elements of this example are connected in tandem
by changing the pitch of grating and the ratio of ridge to groove,
an integrated photodetector which allows a simultaneous detection
of signal lights having a plurality of wavelengths can also be
made.
FIG. 17 is a view showing the fifth example according to this
invention. This example has a horizontal-type p-i-n structure for
photodetection.
The structure of this example has in sequence, coated on the
substrate 71 of semi-insulating GaAs, a buffer layer 72 of
thickness 0.5 .mu.m consisting of i-GaAs, a cladding layer 73 of
thickness 1.5 .mu.m consisting of i-Al.sub.0.5 Ga.sub.0.5 As, a
guiding layer 74 of thickness 0.2 .mu.m which is made a multiple
quantum well (MQW) by laminating a i-GaAs layer of thickness
50.ANG. and a Al.sub.0.5 Ga.sub.0.5 As layer alternately, a
cladding layer 75 of thickness 0.75 .mu.m consisting of
i-Al.sub.0.5 Ga.sub.0.5 As, and a light absorption layer 76 of
thickness 0.3 .mu.m consisting of i-GaAs. Then in the same process
as in the fourth example as shown in FIG. 15, the corrugation-like
grating 77 of 0.05 .mu.m depth tapered to the intensity of coupling
is made on an upper surface of the light absorption layer 76. The
pitch of corrugation is 4.6 .mu.m, and the length of the grating
area is 200 .mu.m. Thereafter, a cladding layer 78 of thickness 1.5
.mu. m consisting of i-Al.sub.0.5 Ga.sub.0.5 As is grown, and
further a protective layer 79 consisting of Si.sub.3 N.sub.4 is
coated.
Then, Zn and Si are placed by thermal diffusion on both sides with
a 2 .mu.m space on an upper surface of the protective layer 79, and
a p-type region 80 and an n-type region 81 are formed.
Subsequently, a cap layer 82 consisting of p.sup.+ -GaAs and an
electrode 83 consisting of Cr/Au are created on the upper portion
of the p-type area 80, while a cap layer 84 consisting of n.sup.+
-GaAs and an electrode 85 consisting of Au-Ge/Au are created on an
upper portion of the n-type area 81.
A reverse bias is applied to the p-i-n horizontal-type structure
thus made, and the wavelength characteristic of detected intensity
for the incident light is observed in the same way as in the fourth
example. As a result, an excellent wavelength selectivity can be
obtained as in the fourth example.
As the structure of this example uses a semi-insulating substrate
71, it is easy to electrically isolate it from other elements, and
it is also advantageous for the integration of a plurality of
photocouplers, or integration with an amplifier for detection, a
light emitting element, or a control driver.
FIG. 18 is a view showing the construction of the sixth example
according to this invention. This example has an amplifying feature
with the FET structure in addition to the wavelength branching
detection feature.
In the same way as the fifth example, this example has coated in
sequence, with the NBE method, on the substrate 91 of
semi-insulating GaAs, a buffer layer 92 of thickness 0.5 .mu.m
consisting of i-GaAs, a cladding layer 93 of thickness 1.5 .mu.m
consisting of i-Al.sub.0.5 As, a guiding layer 94 of thickness 0.2
.mu.m constructed in the same way as the guiding layer 74 of the
fifth example and, a cladding layer 95 of thickness 0.6 .mu.m
consisting of i-Al.sub.0.5 As. Then, using the same process as in
the fourth example, the corrugation like grating 96 tapered to the
intensity of coupling is formed, and subsequently, after the light
absorption layer 97 of thickness 0.4 .mu.m consisting of n-GaAs
(dopening density-1.times.10.sup.17 cm.sup.-3) is regrown, an
insulating layer 98 of thickness 0.3 .mu.m consisting of Si.sub.3
N.sub.4 is coated with the sputtering method.
Then, as shown, a source electrode 100, a gate electrode 101, and a
drain electrode 102 are created on the light absorption layer 97 to
make the FET structure. The source electrode 100 and the drain
electrode 102 are formed from the Au layer having an undercoating
layer of Au-Ge, and the gate electrode 101 is formed of Al.
The operation of this example is performed in the same way as
previous examples, such that the light incident upon the guiding
layer 94 is converted in mode on the grating region 96, and
absorbed in the light absorption layer. Carriers resulting from the
absorption are amplified and detected as a drain current.
Since this example has an amplifying feature with an FET structure
added, in addition to the wavelength detection feature, a
photodetector with an excellent detection sensitivity can be
obtained.
Also in this example, the layer where the grating 96 is formed may
be any region where the guided modes (the even mode 12 and the odd
mode 11) that stand on the light absorption layer 97 or the guiding
layer 94 as the central portion will overlap.
FIG. 19 is a view showing the construction of the seventh example
according to this invention, and FIG. 20 is a cross-sectional view
taken along line A--A'.
This example has grown in sequence, on the substrate 111 of n-GaAs,
a buffer layer 112 consisting of n-GaAs, a first cladding layer 113
of thickness 1.5 .mu.m consisting of n-Al.sub.0.5 Ga.sub.0.5 As, a
guiding layer 114 of thickness 0.2 .mu.m which is constructed a
multiple quantum well (MQW) by laminating non-dope GaAs and
Al.sub.0.5 Ga.sub.0.5 As alternately, a second cladding layer 115
consisting of n-Al.sub.0.5 Ga.sub.0.5 As, and an active layer 116
of thickness 0.4 .mu.m which is made a multiple quantum well (MQW)
by laminating non-dope GaAs and Al.sub.0.4 Ga.sub.0.6 As. For the
crystal growth in this example, the metal organic chemical vapor
deposition (MO-CVD method) was used, but the molecular beam
epitaxial method (MBE method) can be used. The active layer 116 is
formed, and then with photolithography, two gratings 117, 118 are
formed in spaced portions on its upper surface so as to be
appropriate for the wavelength of signal light for light
amplification and to have the same selected wavelength. Then, a
third cladding layer 119 of thickness 1.5 .mu.m consisting of
p-Al.sub.0.5 Ga.sub.0.5 As, a cap layer 120 of thickness 0.2 .mu.m
consisting of p.sup.+ -GaAs, and an insulative layer 121 are formed
in sequence on the upper portion of this photocoupler, and further,
a p-type electrode 122 is provided at the portion corresponding to
the area between the grating 117 and 118 on the upper surface of
the cap layer 120, and an n-type electrode 123 is provided on the
back surface of the substrate 111. The liquid phase epitaxial
method (LPE method) was used to make the third cladding layer 119
and the cap layer 120, but the CVD method can also be used.
Then, in order to make the waveguide 114 a three-dimensional
structure, the both sides of the waveguide 114 are removed with the
wet etching until the first cladding layer 113 is reached, as shown
in FIG. 20, and a buried layer 125 consisting of p-Al.sub.0.5
Ga.sub.0.5 As and a buried layer 126 consisting of n-Al.sub.0.5
Ga.sub.0.5 As are grown in that removed area, with the LPE method,
to form the buried structure.
In this example having two-layer waveguides (waveguide 114, active
layer 116) as shown above, the incident light 128 comprising a
plurality of laser lights having wavelengths ranging from 0.80
.mu.m to 0.86 .mu.m at intervals of 0.001 .mu.m is coupled for
input to the waveguide 114. In this example, in order to perform
the wavelength filtering with light of 0.83 .mu.m wavelength as the
central wavelength, the ratio of ridge to groove changes, in
accordance with the Blackman function distribution as shown in FIG.
11, along the progressive direction of light, with the pitch
.LAMBDA.=9 .mu.m from the expression (1) and the complete coupling
length L=250 .mu.m from the expression (6).
The signal light transferred from the waveguide 114 to the active
layer 116 by the grating 117, the coupling coefficient of which
changes in the propagation direction of light, is amplified during
propagation since the active layer 116 under the electrode 122 is a
laser amplification section having a gain. The signal light
amplified in propagating through the active layer 116 is coupled to
the waveguide 114 again, with the grating 118 formed on the active
layer 116, and output from the input/output waveguide 114, as
previously described.
Thus, by injecting the current only between the grating 117 and the
grating 118, the region of the active layer 116 (except for that
interval) becomes an absorption waveguide, so that the signal input
which is unnecessary for the amplified signal light is eliminated,
and spontaneous emission light having a wavelength other than that
of the amplified signal can also be removed. In this example, even
if the region except for a portion between the gratings 117, 118 is
removed with etching, the same effect can be obtained.
FIG. 21 and FIG. 22 are cross-sectional front and side views,
respectively, showing the construction of the eighth example
according to the invention. In this eighth example, the light
confinement in the transverse direction, which was performed by the
buried layers 125, 126 in the seventh example, is performed by
removing both sides with etching. The construction of this example
is nearly the same as that of the seventh example, wherein like
reference numbers designate like parts.
In this example, a third cladding layer 119 is created, and then
the three-dimensional waveguide 116 is formed as shown by etching
both sides with the reactive ion etching method (RIBE of RIE
method) until the second cladding layer 115 (or first cladding
layer 113) is reached. An impurity diffusion layer 131 formed by
applying the thermal diffusion of the p-type impurities, which is a
conductive type opposite to that of the third cladding layer 119,
on the etching end surface, and an impurity diffusion layer 132 is
formed by applying the same thermal diffusion on the input/output
end surface of the active layer 116, as shown in FIG. 22.
The impurity diffusion layers 131, 132 are intended to disorder
both ends of the active layer 116 and the waveguide 114. The reason
for this will be explained in the following. If a three-dimensional
waveguide (as in this example) is formed, the injection carriers
are recoupled via an interfacial level since a great number of
interfacial levels exist on both end portions of the active layer
116. Consequently, invalid injection carriers increase, and the
signal light transferred from the waveguide 114 is absorbed.
In this example, the super-lattice is disordered on a part of the
active layer 116 and the waveguide 114, in the direction normal to
the propagation direction of light, by the impurity diffusion layer
131, and the super-lattice is disordered on a part of the active
layer 116 in the direction parallel to the propagation direction of
light by the impurity diffusion layer 132. As a result, unnecessary
light is not input into the active layer 116, and at the same time,
the spontaneous emission light other than the wavelengths of
signals that occur in the amplification region can be scattered,
whereby the spontaneous emission light is prevented from emitting
outside along with the amplified signal light.
In this way, this example allows the growth of the crystal to be
made less than the seventh example, and to make a fine adjustment
because the width of waveguide can be controlled by the time of
thermal diffusion.
FIG. 23 is a view showing the construction of the ninth example
according to this invention. This example has coated a substrate
141, a buffer layer 142, a first cladding layer 143, a waveguide
144, and a second cladding layer 145, all with GaAs and AlGaAs to
which impurities are not doped, unlike the seventh and eighth
examples. After coating, the grating (not shown) is formed on an
active layer 146, and a n-type third cladding layer 147 and a cap
layer 148 are regrown. Thereafter, a Si.sub.3 N.sub.4 film is
formed as a diffusion mask, and further, a stripe of about 6 .mu.m
width is formed by photolithography, and the n-GaAs cap layer 148
is etched selectively with an etching liquid consisting of aqueous
ammonia and hydrogen peroxide. Then ZnAs and a sample are sealed in
vacuum, heated at 650.degree. C. for 2.5 hours, and placed in
thermal diffusion on a portion where the cap layer 148 on both
sides is formed, so that an impurity diffusion layer 149 is formed.
The diffusion front at this time may reach to the first cladding
layer 143, and both the waveguide 144 and the active layer 146 are
disordered by Zn, so that the three-dimensional waveguide is
formed. After that, a diffusion mask is removed, a p-type layer
diffused on the cap layer 148 is removed by etching, and an
insulative film (SiO.sub.2) is formed after a p-type electrode 151
is formed. Further, on the insulative film a through hole is
formed, with the photolithography, on which an n-type electrode 152
is formed.
The performance of the device in this example does not differ
greatly from those of the seventh and eighth examples. However,
this example offers more freedom in designing the devices and
fabricating the optical integrated circuits, as non-doped layers
are used up to the active layer 146.
In this way, the interval between waveguides can be set precisely
by fabricating two waveguides (the active layer 146 and the
waveguide 144) in the film direction during the growth of crystal.
Also, since the grating is such that the ratio of ridge to groove
changes along the progressive direction of light in accordance with
the distribution of a specific function, the cross talk between the
waveguide 144 to which the incident light is input and the active
layer 146 can be reduced. As the design of crystal growth or
grating is made easier, optimization of the device can be obtained.
As described above, in the seventh to ninth examples, the
input/output waveguide and the active layer were MQWs that were
super-lattices, but an ordinary thin film waveguide is satisfactory
as well. A structure in which the input/output waveguide is
provided on the active layer is also possible.
In order to perform the optical amplification for a plurality of
wavelengths, a plurality of pairs of gratings having a plurality of
different periods and tapered to the intensity of coupling (each
pair having a different selective wavelength) can be provided.
As described above, when the grating is formed by the corrugation,
it is sufficient that the depth of the groove (low refractive index
region) is increased if the coupling length is required to be
shorter. However, if the depth of groove is deepened, the effective
layer thickness (mean value of layer thicknesses) for the waveguide
formed with the grating, for example, the second waveguide 6 as
shown in FIG. 4, changes largely together with the change of the
duty ratio. The change of this effective layer thickness has an
effect on the selectiveness of wavelength for the guided light. An
example according to this invention which further improves the
above mentioned inconvenience will be explained in the
following.
In the following example, the duty ratio of the grating (the ratio
of a high refractive index region to a low refractive index region
in one cycle of the grating) is changed in the propagation
direction of light, and one pitch (length of one cycle) of the
grating is also gradually changed in the propagation direction of
light. Thereby the change of the effective layer thickness in the
waveguide where the grating is formed can be minimized and an
optical device that allows the effective filtering of a desired
light can be obtained.
FIG. 24 is a partial cross-sectional perspective view showing the
tenth example according to this invention. In the same figure, this
example has grown in sequence with the molecular beam epitaxy (MBE)
method, on the substrate 201 of GaAs, a buffer layer 202 of
thickness 0.5 .mu.m consisting of GaAs, a cladding layer 203 of
thickness 1.5 .mu.m consisting of Al.sub.0.5 Ga.sub.0.5 As, a first
waveguide 204 which is made a multiple quantum well (MQW) by
laminating each of nine GaAs layers (30.ANG.) and Al.sub.0.5
Ga.sub.0.5 As layers alternately a cladding layer 205 of thickness
0.8 .mu.m consisting of Al.sub.0.5 Ga.sub.0.5 As, and a second
waveguide 206 consisting of a multiple quantum well (MQW)
constructed by laminating 55 GaAs layers (30.ANG.) and Al.sub.0.4
Ga.sub.0.6 As layers (60.ANG.), alternately, and the Al.sub.0.4
Ga.sub.0.5 As layer (300.ANG.) on the upper portion of MQW.
Then using a photoresist, a stripe of 12 .mu.m in depth is formed,
a wafer is etched in a stripe-like form up to the GaAs buffer
layer, with an etching liquid containing a mixture of sulfuric
acid, hydrogen peroxide and water. After removing the photoresist,
the Al.sub.0.4 Ga.sub.0.5 As layer 207 is regrown with the liquid
phase growth, and the stripe is embedded. In this case, as an upper
portion of the stripe is the Al.sub.0.4 Ga.sub.0.6 As layer
(uppermost layer of the second waveguide 206), the Al.sub.0.5
Ga.sub.0.5 As is not regrown.
Subsequently, the grating is formed in the direction orthogonal to
the above mentioned stripe, using a photoresist. Also, using an
etching liquid containing a mixture of sulfuric acid, hydrogen
peroxide and water, the second waveguide 206 is etched partially,
and the grating consisting of corrugation of 700.ANG. in depth is
formed. Thereafter, with the sputtering method, SiO.sub.2 is coated
on the grating 208. The back surface of wafer is wrapped so that
the thickness of wafer may be made 100 .mu.m. Furthermore, this
wafer is cut to a predetermined size, and ZrO.sub.2 is deposited on
the input/output end face of cut water to make an unreflective
coating. Thus the structure with two waveguides 204, 206 laminated
as shown in FIG. 24 can be obtained.
In this example, in order to suppress the side lobe filter
characteristic, the coupling coefficient of the grating 208 is
gradually changed over the coupling area. The distribution of the
coupling coefficient changes stepwise depending on the position
within the coupling area (shown with a value z normalized by the
complete coupling length), as shown with a bold solid line in FIG.
25. The modified embodiment is constructed almost symmetrically
along the traveling direction of light from the central portion
toward the input and output sides of light. A fine line in FIG. 25
indicates the coupling coefficient of the Hamming function
F(z)=1+0.852.multidot.cos (2.pi.z/L). The stepwise coupling
coefficient of the bold line almost approximates the fine line and
can provide in practice the same effect as the Hamming function for
the distribution of the coupling coefficient.
Such a distribution of the coupling coefficient can be obtained by
appropriately distributing the duty ratio (ratio t/.LAMBDA.
occupied by the ridge width t in a pitch .LAMBDA.) and the pitch
.LAMBDA. of the grating 208 for a specific selected wavelength (in
the following explanation, .lambda..sub.0 =0.83 .mu.m).
FIG. 26 shows the pitch and the coupling coefficient to the duty
ratio t/.LAMBDA. of the grating, when the specific selected
wavelength .lambda..sub.0 is selected. The pitch tends to be
smaller if the duty ratio is greater, while the coupling
coefficient is at maximum near the duty ratio of 0.8. This is
explained in the following way. If the duty ratio is increased, the
growth width (portion of groove) of the grating is narrowed, so
that the effective thickness of the second waveguide 206 is
thicker, and the value of the propagation constant .beta..sub.even
for the even mode of light propagating through the second waveguide
6 is greater. Assuming the propagation constant for the odd mode of
light propagating through the first waveguide 4 to be
.beta..sub.odd' the pitch .LAMBDA. of the grating 208 for realizing
the phase matching between the even mode and the odd mode can be
given by the following expression.
Therefore, if .beta..sub.even is greater, the pitch .LAMBDA. is
smaller.
In order to obtain the distribution of the coupling coefficient as
shown in FIG. 25, a combination of the duty ratio and the pitch at
which a desired coupling coefficient can be obtained at each point
of the coupling area is selected from FIG. 24. As a result of such
a selection, the duty ratio changes over the entire coupling area,
as shown in FIG. 27, to establish the distribution of the coupling
coefficient of FIG. 25. The pitch is distributed over the entire
coupling area, as shown in FIG. 28.
FIG. 29 shows the result of measuring the output light intensity
(or the coupling efficiency where the ratio of the intensity of the
incoming light at each wavelength to that of the outgoing light is
expressed in dB units) from the second waveguide 206, after
introducing the light from a tunable light source into the first
waveguide 204 of the optical wavelength filter. Thereby it can be
seen that the optical wavelength filter of this example has a
sufficiently suppressed side lobe, and has low cross talk between
wavelengths.
On the other hand, FIG. 30 shows the intensity of output light from
the first waveguide 204, indicating the notch filter characteristic
in which the light does not transmit only at the central wavelength
.lambda..sub.0.
Also, the depth of the grating was 700.ANG. in this example, but
when the grating depth is 200.ANG., for example, the pitch to the
duty ratio is almost constant as shown in FIG. 31. Therefore, even
though the duty ratio is distributed functionally, the coupling
coefficient can be distributed as shown in FIG. 25, to obtain an
excellent filter characteristic.
On the contrary, FIG. 32 shows a case where the grating depth is
deeper such as 1000.ANG.. As seen, if the grating depth is
increased, the pitch must be distributed in a tapering manner to
obtain an excellent filter characteristic.
In this way, it should be understood that the distribution of pitch
must be considered for a grating depth of 700.ANG., while it does
not have to be a primary consideration for the grating depth of
200.ANG., in the example as shown in FIG. 24. But an appropriate
grating depth largely depends on the layer thicknesses or
refractive indices for the waveguides and the cladding layers
constituting the filter. Therefore, in the construction of an
optical device, the duty ratio and the distribution of pitch must
be determined so as to attain an excellent filter characteristic
corresponding to the grating depth.
On the contrary, when the pitch is to be changed, an excellent
filter characteristic can be obtained, as shown in FIG. 25, by
appropriately selecting the distribution of the pitch and the duty
ratio, even though the grating depth is increased to shorten the
coupling length.
FIG. 33 shows how the coupling length changes when the grating
depth is changed. It will be seen that if the grating depth is
larger, the coupling coefficient on the coupling area becomes
larger, and the coupling length (the length of waveguide required
to completely transfer the optical power between waveguides)
becomes shorter. As short coupling length is an important factor
for integration, the grating depth must be increased if required.
According to this invention, even when the grating depth is large,
the side lobes can be effectively suppressed by making the
distribution of pitch taper-shaped.
Here the operation of the example shown in FIG. 24 is described.
The incident light 210 multiplexed in wavelengths ranging from 0.80
.mu.m to 0.86 .mu.m is coupled for input to the first waveguide 204
of this example. There are two guided modes which stand in two
waveguides 204, 206, i.e., the even mode and the odd mode. The
incident light 210 to the first waveguide 204 propagates in the odd
mode on the first waveguide 204 as the central portion. Then as the
odd mode and the even mode have different propagation constants,
the light propagates almost without coupling in the region where
the grating 208 does not exist. However, in the region where the
grating 208 exists, the transfer of optic power of wavelength
.lambda. occurs if the above expression (9) is satisfied between
the propagation constant .beta..sub.odd (.lambda.) for the odd mode
and the propagation constant .beta..sub.even (.lambda.) for the
even mode.
If the transfer of optic power occurs, the guided light in the odd
mode to which the incident light 210 is coupled can be converted
into the guided light in the even mode. Accordingly, the incident
light 210 finally becomes the optical wave propagating through the
second waveguide 206, and is output as the selected output light
211. The light containing other wavelengths is output from the
first waveguide 204 as the unselected output light.
FIG. 34 shows a schematic view of the eleventh example according to
this invention, when the distribution of the coupling coefficient
.kappa. is a Gaussian distribution. The present example is an
optical wavelength filter that was created by the same process as
the tenth example. The result of measuring the transmitted spectrum
from the second waveguide 206 according to this example is shown as
a solid line in FIG. 35. Compared with a case of uniform coupling
coefficient distribution (indicated with a dashed line), the side
lobe is suppressed by 20-25 dB. The cross talk between the central
wavelength (.lambda.=0.83 .mu.m) and the wavelength 100.ANG. off
that central wavelength exceeded -35 dB. In FIG. 35, the dashed
line indicates the filter response of a comparative example when
the coupling coefficient is uniform over the entire coupling
area.
The examples shown in FIG. 34 and FIG. 35 can be applied to a noise
filter integrated with an optical amplifier using a laser
structure, an integrated internal filter of an external resonator
type laser, or a filter integral with an optical filter. These
examples can be formed of III-V group compound semiconductor except
for GaAs group, or II-VI group compound semiconductor such as CdTe,
or SiO.sub.2, Si.sub.3 N.sub.4, LiNbO.sub.3.
* * * * *